1. Field of the Invention
The present invention relates generally to position-encoding devices (generally abbreviated xe2x80x9cencodersxe2x80x9d), particularly rotary encoders (also sometimes referred to as xe2x80x9ccode wheelsxe2x80x9d). Even more specifically, the present invention provides rotary encoder devices useful for ink-jet hard copy apparatus where accurate positioning of motor driven subsystems, such as pen servicing elements and transports for accurately advancing the printing medium through a printing-zone, is required.
2. Description of Related Art
An encoder generally has two main elements that are subject to relative movement. One of the elements is extended along the direction of relative movement that is to be measured and has marked graduations that are, in effect, arrayed along that direction of movement. The other element is positioned to sense relative passage of such graduations and in response to produce a signal indicative of the movement and which can be translated into a position reckoning. Both linear and rotary encoders are known in the art. xe2x80x9cResolutionxe2x80x9d is the ability of the system to properly and reliably distinguish each graduation from adjacent ones. This ability may also be described as the readability of the graduations through interpretation of the signals transmitted, or the precision with which the signals correspond to the passage of graduations passed the sensing element. A typical encoder subsystem comprises a optical detector device (reflective or transmissive) mounted in conjunction with a graduated encoder disk acting as a light interrupter to generate signals indicative of the relative position.
Ink-jet printers create text and images as arrays of a very large number of computer-controlled placements of individual, minute, ink droplets on a printing medium that is computer-control advanced in very small steps through the printer during the ink deposition process. Encoders are used to determine positioning of print media relative to the ink jet writing instrument. Other subsystems of an ink-jet printer, such as the positioning of movable service station elements, can employ relatively low resolution encoders directly mounted to a DC drive motor.
For example, rotary encoders can be used to establish image positioning on a print medium as shown in FIG. 1 (Prior Art) herein from U.S. Pat. No. 5,598,201 filed on Jan. 31, 1994, and by Stodder et al. for a DUAL-RESOLUTION ENCODING SYSTEM FOR HIGH CYCLIC ACCURACY OF PRINT-MEDIUM ADVANCE IN AN INKJET PRINTER (issued Jan. 28, 1997, assigned to the common assignee and incorporated by reference herein).
Referring to FIG. 1 (Prior Art), two rotary encoders 41, 51 are linked through a gear train 21. The gear train 21 consists of a spur 22 on a shaft 12 of the print-medium drive platen/roller 11, and a pinion 23 that engages the spur 22 and rides on a shaft of a motor 31. The motor 31, train 21 and roller 11 together advance 13 a piece of printing medium 1 longitudinally relative to a printhead, or pen 71. The pen 71 is mounted for transverse motion 72 to mark on the medium 1 at coordinate positions established orthogonally by the medium advance 13 and pen motion 72. Each encoder 41, 51 includes a respective encoder disk 42, 52 and encoder sensor 43, 53. One encoder disk 42 is on the platen shaft 12 and the other disk 52 is on the motor shaft 32. Accordingly, the encoder disk 52 has a mechanical advantage relative to the platen 11. In operation, encoder signals 44, 54 from respective encoders 41, 51 are transmitted to digital electronic controller for 61 for processing which in turn provide signals 62 to control the platen 11 motion and pen 71 motion 72. Relatively low resolution disks 42, 52 can be employed by use of quadrature signal generation and digital signal processing techniques.
Commercially available products such as the Hewlett-Packard(trademark) DeskJet(trademark) models 720 and 722C incorporate an optical encoder wheel on the axle of the print media advance tires. Note that in addition to the illustrated mechanical type paper transport subsystem, endless-loop, vacuum belt, print media transports are also employed in ink-jet printers for moving the media through a printing-zone; the belt position in the printing zone at a given time relative to a printhead is critical to image formation. It will also be recognized by those skilled in the art that document scanners and other hard copy apparatus such as facsimile machines that may incorporate scanning subsystems may use the same type media advance system as a simple computer printer. The present invention may be employed in such other types of hard copy apparatus where incremental advance of the media must be coordinated with other subsystems.
In printers, the accuracy of position determinations along the advance direction of the medium is limited by the positional accuracy of the encoder system graduations. In fact, in printing with a digitally manipulated matrix of very small droplets of ink, for example at a resolution of 1200 dots-per-inch or greater, precision in alignment of the print media with the printhead is a key factor of resultant print quality.
In general, opaque disks used for encoders are preferably made of metal, providing sturdiness of construction, punched through with light transmitting slits. Metal encoder disks are relatively expensive. Moreover, metal type encoder disks are inherently a low resolution mechanism as punching processes attempting to form high resolution graduations can bend or deform the material. Chemical etching or precision laser cutting of the slits are relatively expensive manufacturing techniques.
In other known rotary encoders, a printed film technology is employed. Printed films can achieve very precise encoder lines through photolithographic processes performed on a thin plastic film. Thus, relatively high resolution position encoding can be achieved. However, these films employed are generally too thin to attach directly to a rotating shaft. Therefore, a separate piece part for a hub and attachment manufacturing step is required. Thus, this is a moderately expensive solution.
Moreover, the printed graduation marks can be scratched from stamped plastic, and to a lesser extent on films, leading to encoder subsystem failures.
Heavier plastic disks with molded through holes are difficult to achieve due to the precise mating of mold halves required to achieve holes without flash. Moreover it is difficult to push material in to fill apertures where desired. Thus, through hole type encoders are capable of only relatively low resolution.
Generally, it is also known in the art to provide relatively thick, transparent material, encoder disks with graduation marks using silk screening or photochemistry. The known processes for simply printing a pattern on an inflexible surface also has problems. Pad printing of lines results in varying line thicknesses depending on ink quantity and pressure; controlling run-out (the abbreviation for tolerance problems known to occur in rotating disk technologies) presents difficulties for printing alignment; smudging and voids can occur, reducing the process yield. Hot stamping, or coated foil transfer, using a heated die in the shape of the desired graduation marks, presents the same issues; moreover, ink bridging the small gap between marks can occur.
Therefore, there is a need for providing a relatively low cost encoder having equivalent resolution capabilities to that provided by the prior art.
For convenience in describing the art and the present invention, all types of ink-jet hard copy apparatus are sometimes hereinafter referred to as xe2x80x9cprinters;xe2x80x9d all types, sizes, and compositions of print media are also referred to as xe2x80x9cpaper;xe2x80x9d all compositions of colorants are sometimes referred to as xe2x80x9cink;xe2x80x9d and all embodiments of an ink-jet writing instruments are simply referred to as a xe2x80x9cpen;xe2x80x9d no limitation on the scope of the invention is intended nor should any be implied.
In its basic aspects, the present invention provides an encoder disk including: a molded, unitary polymer construct having a disk portion with alternating, uniform, radial ridges and valleys on a first disk surface, wherein the ridges form position encoding graduations.
In another basic aspect, the present invention provides a method for forming encoder disks, including the steps of: molding a unitary body, polymer disk, and molding one surface with a circumference defined by a series of radially extending, alternating, ridges and valleys.
In another basic aspect, the present invention provides a paper position detecting device for an ink-jet printer having a paper transport mechanism, the detecting device including a light detector mechanism, including: coupled to the paper transport mechanism, a rotary encoder disk including a unitary, polymer construct having a disk portion with alternating, uniform, radial ridges and valleys on a first disk surface, an aperture defining a rotational axis of the disk, and on a second surface of the disk, a shaft attachment portion, axially coincident with the aperture, for receiving a complementary member of the paper transport mechanism, wherein the ridges are reactive to transmitted light from the light detector mechanism, forming position graduations related to print media positions established by the paper transport mechanism.
Some of the advantage of the present invention are:
it provides a low cost, high reliability, solutions to problems inherent in equivalent resolution, prior art, encoders;
it has a lower cost than printed film encoder types;
it is a unitary piece part, lower manufacturing costs and device complexity;
it reduces manufacturing tool and complexity requirements;
it is resistant to handling damage;
it can be directly mounted to a rotating shaft;
in a second embodiment described hereinafter, high resolution rotary encoderxe2x80x94comparable to prior art thin film resolutionxe2x80x94is achieved in a low cost, unitary construct capable of direct mounting to a shaft;
in the second embodiment described hereinafter, it provides an encoder disk having higher resolution than through hole encoder types; and
in the second embodiment described hereinafter, it provides a versatile graduation marking capability. it reduces run-out problems;
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.